Frequent mutation of Apc gene in rat colon tumors and mucin-depleted foci (MDF), preneoplastic lesions in experimental colon carcinogenesis

Mucin-depleted foci (MDF) are microscopic dysplastic lesions induced in the colon of rodents by specific colon carcinogens. Most MDF show Wnt pathway activation whereas only a subset shows mutations in the Ctnnb1 gene, coding for β-catenin. Since Apc is a member of the Wnt pathway and the most frequent mutated gene in human colon cancer, we tested whether MDF harbor Apc mutations. F344 rats were treated twice with 150 mg/kg of 1,2-dimethylhydrazine (DMH). Fifteen or 28 weeks later MDF, Aberrant Crypt Foci (ACF) and tumors were collected. We screened a segment of the Apc gene, comprising the region homologous to the Mutation Cluster Region (MCR) of human APC, which frequently shows mutations in experimental colon tumors. Mutations were identified by PCR amplification and sequencing in 6/24 MDF (25%), 7/23 tumors (30%), 0/24 ACF (0%). Most of the mutations (92%) in MDF and tumors were localized in a region upstream from the MCR. All mutations were single base substitutions and mainly formed by G:C → A:T and C:G→ T:A transitions. The pattern of nucleotide changes was similar in MDF and tumors and, interestingly, the same mutation in codon 1047 was found in 2 MDF and in 3 tumors. Four out of the six mutations found in MDF were nonsense mutations and two were missense. All mutations in tumors determined a protein truncation. These results show that Apc mutations are present in MDF with a frequency similar to that of tumors, strengthening the evidence that they are precancerous lesions in colon carcinogenesis.